Experimental Investigation on Lean Blow Out of a Piloted Aero-Engine Burner

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One of the preferred ways to reduce NOX formation in an aero-engine is to operate lean throughout the whole operational range; however the lean combustion suffers from poor stability. To avoid the problem associated with stability, often a rich pilot flame is used along with a main flame to act as a source of heat and radicals to the main flame. The focus of the paper is to discuss the influence of the liquid fuel spray characteristics and effect of flow parameters on the lean blow out (LBO) limits of a piloted burner. In order to understand the observed remarkable LBO limits of the pilot flame with Jet A-1 (LBO = 145 kg-air to kg-fuel at 0.1 MPa of combustor pressure), velocity field measurements by laser Doppler Anemometry (LDA) technique have been performed. Furthermore, the flame structure has been analyzed with OH* chemiluminescence imaging. Experimental results show that the LBO limits of the burner running with liquid fuel further improves with an increase in combustor pressure. Such improvement in LBO limits is attributed to the change in the liquid fuel distribution caused by the change in the combustor pressure. For gaseous fuel measurements, results indicate that the equivalence ratio and the momentum ratio of the pilot jet to the co-annular flow are the dominating parameters that control the mixing process in the combustor and the ensuing effect on the flame structure and location of flame stabilization is substantial. The flame stabilizes either along the centreline or along the shear layer between two jets. Such information is useful in designing a lean partially premixed combustion system where a pilot flame is required to stabilize a main lean flame.